There are a number of methods for the preparation of tantalum metal powder by reduction of tantalum halides or halide double salts with alkali metals. All these methods yield tantalum powders that are suitable for certain purposes for example, for the production of sintered bars for the manufacture of tantalum wire or plates. Several of these methods were developed expressly for the preparation of tantalum metal powder for the manufacture of electrolytic capacitors. Niobium metal powder is produced in similar manner.
According to German Pat. No. 1,126,144, K.sub.2 TaF.sub.7 is reduced with sodium in charges in a vacuum or in an atmosphere of a protective gas and, after the usual wet-chemical preparation of the reaction cake, the coarse portion over 150 .mu.m and the fine portion under 32 .mu.m are separated from the tantalum metal powder. The separated fine portion is heated under high vacuum at 700.degree. to 900.degree. C. to increase the grain size and the portion is added to the main fraction. Tantalum powders prepared this way yield capacitances of 2700 .mu.C/g when processed to capacitor anodes.
U.S. Pat. No. 3,647,415 describes the preparation of a tantalum powder for electrolytic capacitors with a certain grain form of the tantalum crystals. The attainable capacitance of these powders is 4000 .mu.C/g.
U.S. Pat. No. 3,829,310 describes an improved process where a mixture of well-dried potassium tantalum fluoride and an alkali metal halide is coated with liquid sodium up to 300.degree. C. and the resulting mixture (which has a considerably lower temperature and for which a spontaneous ignition temperature of minimum 270.degree. C. is indicated) is ignited in a reaction vessel. After the ignition, the temperature rises to about 1000.degree. C. With additional heating this temperature is maintained for one hour. When the grain fraction of this tantalum powder which is over 5 .mu.m but less than 45 .mu.m is used for the protection of test anodes (sintering temperature 1,600.degree. C.), the anodes have a .mu.C/g value of up to 9,500 with a residual current of about 15 .mu.A/g. The portion of this grain fraction is only up to 32% of the total powder; for this reason this process is uneconomical for the preparation of high capacity tantalum powders.
Other processes for the preparation of tantalum powder, which are not geared specifically to the capacitor quality, comprise substantially simpler steps, and are also listed here.
U.S. Pat. No. 2,950,185 describes a process where liquid K.sub.2 TaF.sub.7 is reduced under stirred at temperatures above 800.degree. C. to fine-grained tantalum-metal powder by adding liquid sodium. The grain distribution of the tantalum powder obtained is similar to the tantalum of U.S. Pat. No. 3,829,310.
According to U.S. Pat. No. 3,012,877 the reduction is effected continuously in the molten state. The molten state leads preferably to the formation of coarse tantalum particles, which has the result that the powders obtained are only to a limited extent or not at all suitable for the production of high capacity tantalum capacitors.
All presently known methods have the principal disadvantage that both the course of the reduction reaction in larger units and the resulting temperature accumulation and the holding times at high temperatures required to complete the reduction have the effect that it is not possible to obtain tantalum powders which have maximum capacitance values suitable for use as capacitors. It is a further object to avoid these disadvantages.
In this specification:
".mu.m" stands for micrometer or micron,
".mu.C/g" stands for microcoulomb per gram, and
".mu.A/g" stands for microampere per gram.